Dielectric switching of the nature of excited singlet state in a donor-acceptor-type polyfluorene copolymer

The spectral evolution of an intrachain neutral singlet exciton toward a charge-transfer (CT) state in solvents of increasing polarity has been monitored by time-resolved photoluminescence and ultrafast transient-absorption spectroscopy in a model conjugated random copolymer composed of electron donor and electron acceptor units. In polar solvents, a charge-like absorption superimposes the region of stimulated emission and leads to a dramatic reduction in gain implying that CT states can be detrimental for light amplification and lasing.